Oil field valves and other fittings are constructed in cast bodies in most instances. The cast bodies are drilled with a number of blind holes at a flange face to enable connection of the flanged fitting into a pipeline or other mating apparatus. The connection is thus made through the use of headless stud bolts. Blind holes are drilled in the facing flange plate and are tapped with a thread to receive a headless stud bolt therein. The headless stud bolt is thus threaded into the tapped blind hole. Thereafter, the upper part of the bolt which incorporates a second threaded portion is utilized with a mating nut to attach a second flange plate to the first flange plate in completing the assembly of the equipment.
Two problems occur from time to time with this kind of construction. First of all, the headless stud bolt, typically comprised of a central unthreaded portion with threads at each end, is difficult to grasp and has a tendency to work loose and become unthreaded. It is difficult to grasp and thereby difficult to thread into the blind hole with the maximum permissible torque to prevent backing out. This can be cured in some measure through the utilization of split ring washers or an adhesive dope applied to the threads. While that will definitely reduce backing out, it is achieved with an increase in labor in assembly of the flange fittings together.
A second problem is leakage along the threads in the tapped blind hole. Often, hairline cracks will radiate to the end of the tapped blind hole from the interior of the casting. Depending on the vagaries of the path, the crack might well develop a means of leakage. The leaked material will then flow along the threads to escape from the fitting. This can be remedied in part by application of thread doping material to reduce communication along the threads. Again, this is achieved at notable added cost.
With these problems in view, the improved headless stud bolt of the present disclosure overcomes these problems by providing a modified thread with the stud bolt. A stud bolt constructed in accordance with these teachings can then be used with a conventionally formed tapped blind hole in a cast flange fitting. When installed in the tapped blind hole, the fitting is sealed against leakage along the threads. This is accomplished by modifying the last two or three turns of the thread. The thread that is formed on the headless stud bolt is defined by a crown which is the top edge. It is also defined by a root which is at the bottom or centered in the valley between adjacent threads. The crown is modified by increasing the width of the crown by at least one hundred percent for threads of usual manufacturing quality. The root is elevated relative to the thread, thereby positioning the root at a point where it jams against the mating crown of the blind hole thread. This nearly creates an interference fit when the last turns of the stud bolt are mated on installation. The crown of increased width jams against the more narrow root of the tapped hole to initiate an interference fit. The raised root also jams, and this jamming occurs preferably for at least one full turn around the stud bolt.
The foregoing described structure has another feature in that it locks the stud bolt in position to thereby prevent unintended backing out. Vibrations over a long period of time may very well back out headless, threaded stud bolts from a tapped blind hole. The interference fit achieved in the last two or three turns prevents backing out.
One advantage of this apparatus is that it can be machined on a conventional thread cutting machine which is characterized as a lathe having a cutting point traveling on a lead screw to thereby form the thread at the desired depth and pitch. While automated machinery may be used to form bolts in quantity, the machine, itself, is not modified and can be used in the intended manner.
In fabrication of the headless stud bolt of this disclosure, the thread end of the bolt to be received in the tapped blind hole is cut in the customary manner, except that the tool point is slightly withdrawn in forming the last two or three turns of the thread. The net result is that the conventional bar stock material used to form the stud bolt is cut in the conventional fashion to the outer cylindrical wall wherein withdrawal of the cutting tool defines the crown to a greater width. The increased width is obtained by the uniform withdrawl of the cutting tool. The same is true of formation of the root at the last two or three turns.
With the foregoing in mind, the apparatus is thereby summarized to incorporate a modified thread which includes about two turns wherein the last two turns which mate with the tapped blind hole are cut with an increased width on the crown and a raised root. The pitch of the thread otherwise remains the same. The crown of increased width can be conveniently obtained by slightly retracting the cutting tool and forming the thread so that the outer surface of the bar stock forms the widened crown.